1. The Field of the Invention
The present invention relates to a process and system for reducing pollutants from the engine exhaust of automobiles and other mobile sources. More particularly, the present invention relates to a process and system for removing oxides of nitrogen (NO.sub.x) in such a way as to simultaneously permit reduction of the emissions of carbon monoxide, unburned hydrocarbons, and other byproducts of internal combustion engine operation.
2. The Relevant Technology
In many parts of the United States, automobiles account for much of the total NO.sub.x burden on the environment. This is especially notable in areas such as southern California where estimates made by the South Coast Air Quality Management District suggest that over half of the total anthropogenic NO.sub.x is due to automobiles. In addition to concerns over acid rain, it has been suggested that failure to adequately control NO.sub.x emissions is also partly responsible for ozone non-attainment in many areas.
Catalytic converters have been used for pollutant removal from automotive exhaust for many years. Increasingly stringent government regulations for the allowable emission levels of carbon monoxide (CO), hydrocarbons (HC), and NO.sub.x have resulted in a majority of gasoline-powered vehicles operated in the United States being equipped with three-way catalytic converters. The "three-way" catalysts in such converters oxidize carbon monoxide and hydrocarbons, while simultaneously reducing NO.sub.x. Thus, three-way catalysts are capable of removing all three of the above pollutants simultaneously, provided that the catalyst is maintained in a chemically correct environment that is neither over oxidizing or reducing.
Catalytic converters are quite efficient at reducing NO.sub.x emissions at normal operating temperatures, but are not effective during the first minute or two after initial engine start-up, when the catalytic converter is essentially cold. It has been estimated that during this warm up period, one-half of an automobile's NO.sub.x emissions are released. Automobile emissions account for about 50% of the total NO.sub.x inventory in many urban areas. Accordingly, if NO.sub.x could be largely eliminated during the warm up period of a catalytic converter, then conceivably up to about 25% of the total NO.sub.x inventory could be eliminated in many urban areas.
During cold start conditions, most automobile engines operate with a fuel-rich stoichiometry in order to reduce NO.sub.x emissions prior to the catalytic converter heating up. While this allows smooth engine operation and sufficient cold power, CO and hydrocarbon emissions increase significantly. Internal combustion engines using newer technology can efficiently operate fuel-lean during cold start conditions. This fuel-lean operation reduces CO and hydrocarbon emissions and slightly reduces fuel consumption. A fuel-lean cold start can also, however, significantly increase NO.sub.x formation.
Air pollution regulations are becoming stricter in most urban areas and as a result, rigid emissions limits are being placed on mobile sources. If "cold start" NO.sub.x start up emissions could be diminished or eliminated, the concurrent reduction of CO and unburned hydrocarbon emissions in newer engines would result in a total reduction of automotive exhaust emissions in urban areas by as much as 85 percent according to some estimates.
The reversible adsorption of NO.sub.x on numerous sorbents, including supported transition metal oxides (M.sup.II O, M.sup.III.sub.2 O.sub.3 or M.sup.II,III.sub.3 O.sub.4) has been demonstrated previously. For example, U.S. Pat. No. 5,362,463 to Stiles et al. discloses a method for preparation of a solid sorbent capable of removing NO.sub.x from gases containing combustion products. This sorbent consists of a transition metal oxide, preferably manganese dioxide, promoted by potassium carbonate and supported on alumina.
Several patents have discussed processes in which contaminant constituents in a gas stream are adsorbed onto a sorbent, and the sorbent is later regenerated to be re-used immediately. These patents include U.S. Pat. Nos. 4,764,187 and 4,849,111 to Abrams, U.S. Pat. No. 4,778,492 to Dawson, and U.S. Pat. No. 4,915,922 to Filss. The regeneration step in each of these patents, however, utilizes chemical desorption. Chemical desorption typically requires that the sorbent regeneration step occurs using a fluid completely free of the contaminant being desorbed, which would not be practical for automotive engine applications.
In U.S. Pat. No. 4,775,484 to Schmidt, a sorption bed system is disclosed that uses a regenerable sorbent to remove a contaminant from a gas stream. This system, however, includes 3 distinct zones, and rotates around an axis to expose the individual zones to the desired gas stream. These characteristics would not be economically or strategically attractive for application to mobile source engine exhaust.
Accordingly, there is a need for a process and system to improve start up emissions from mobile source engine exhaust.